The present invention concerns corrosion-stable aluminum pigments produced by physical vapor deposition and a process for the stabilization thereof.
Pigments which are produced by way of physical vapor deposition (PVD) of single-layer or multi-layer films on a carrier, subsequent detachment and then crushing of the films or film packs are becoming of increasing interest in very recent times because of their special optical properties. Thus for example single-layer aluminum pigment which is produced by way of PVD and which is known under the name METALURE (registered mark) is highly valued in the printing and paints industry because of its outstanding mirror shine while multi-layer so-called optically variable pigments which have brilliant interference colors and which produce pronounced angle-dependent color shade variations are increasingly used in paints, plastic materials and in bond printing.
The single-layer aluminum scales or flakes with a very high degree of shine of the above-mentioned aluminum pigment are produced by vacuum deposition on a substrate provided with a release layer, subsequent detachment of the aluminum film, and mechanical crushing thereof. The thickness of the film particles is generally less than 100 nm. The scale or flake surface is mirror-smooth and is of the highest level of perfection. The flake or scale surface can however also have a hologram-like embossing (WO 93/23481). Depending on the embossing involved such flakes appear in variable colors.
The basic structure of optically variable multi-layer pigments is as follows: following a central highly reflective metal layer M, towards each side, is a transparent low-refractive layer T and then a semi-transparent metal layer Mxe2x80x2. Films which involve a multi-layer structure of the type Mxe2x80x2TM have been known for many years (see Optical Acta 20, 925-937 (1973) and U.S. Pat. No. 3,858,977). Pigments which embody the optical principle which is applicable in respect of films and which have the above-mentioned layer sequence Mxe2x80x2TMTMxe2x80x2 were first described in U.S. Pat. No. 3,438,796. The flake-like pigment particles, envisaged for the area of use of decorative paints and lacquers, exhibit brilliant colors and comprise a highly reflective, central aluminum layer which is at least 60 nm in thickness and which is accompanied in an outward direction by a respective SiO2-layer which is between 100 and 600 nm in thickness and which is then followed by a semi-transparent aluminum layer which is between 5 and 40 nm in thickness. Thereover there is also an SiO2-protective layer. The production of such pigments is effected by vapor deposition of subsequent layers and then crushing of the multi-layer film to the particle size of special-effect pigments. So as to facilitate detachment of the film from the substrate it is covered with a release layer. The color of the pigments produced in that way depends on the thickness of the SiO2-layers. Each color shade of the spectrum can be specifically set by way of the choice of the thickness of the SiO2-layers. Higher-order interference colors are also possible.
Pigments involving a similar structure and a similar production process are described in U.S. Pat. No. 5,135,812 and EP-A227 423. Those pigments have a multi-layer structure, wherein the central opaque layer comprises a highly reflective metal layer, generally aluminum, the transparent layers which follow it in an outward direction comprise MgF2 or SiO2 (refractive index n less than 1.65) and a semi-transparent or semi-opaque metal layer. The area of use is printing inks for forgery-resistant banknotes. The pigments are produced by physical vapor deposition in a vacuum and then crushing of the multi-layer film, generally in an ultrasonic crusher, to pigment particle size.
The pigments described in the above-indicated patents suffer from the disadvantage that they are susceptible to corrosion by virtue of exposed metal surfaces. Admittedly the pigments which are produced from films manufactured by vapor deposition in a multi-layer structure can already be passivated on the large surfaces of the pigment particles by the production of a protective layer in the vapor deposition process, as was effected in accordance with U.S. Pat. No. 3,438,796. The operation of crushing the multi-layer film however also gives rise to fresh fracture locations which, due to the procedure involved, are unprotected and are therefore highly sensitive to corrosion. Particularly in the presence of moisture, acids or bases, the chemical reactivity of the fresh fracture locations results in corrosion and thus inevitably results in an impairment in the brilliance and coloristics of the pigment scales or flakes. That represents a serious problem in terms of technical application. U.S. Pat. No. 5,498,781 described an initial attempt at passivating optically variable pigments for aqueous coating systems. The ready-for-sale, optically variable pigment powder was thereafter surface-coated with a silane compound of the type R3 Sixe2x80x94Axe2x80x94X, specifically (CH3CH2O)3 Si(CH2)3 NH2 in an aqueous alcohol solution, tempered at 110xc2x0 C. and then reacted in an alcohol solution with a polymer bearing functional isocyanate groups.
The result is a finished lacquer or paint as is used for the decorative surface coating of motor vehicles. Thus the passivation operation described in U.S. Pat. No. 5,498,781 leads to a single system of use and lacks the multiple and varied use options of a passivated pigment. A further disadvantage of that passivation process is that the corrosion processes have already begun at the endangered fracture edges of the optically variable pigments when treatment of the finished pigment powder begins.
An object of the present invention is to improve the corrosion characteristics of pigments produced by PVD and subsequent crushing of the films.
Another object of the invention is to provide pigments produced by PVD and then crushing, which remain substantially stable even in aggressive aqueous solutions such as acid or alkaline or solvent-based agents.
A further object of the invention is to provide that fresh fracture locations which are produced by a film-crushing operation, in relation to optically variable multi-layer pigments, more especially fracture locations of a central, highly reflective aluminum layer, are passivated.
A still further object of the invention is a process for the stabilization of aluminum-based metal pigments manufactured by physical vapor deposition and subsequent crushing of the metal film produced.
In accordance with the invention the foregoing and other objects are attained by an aluminum-based metal pigment produced by physical vapor deposition (PVD), wherein all metal surfaces and in particular fracture surfaces of the metal pigment particles, which are exposed after the step of crushing a metal film produced by PVD, are covered with a passivating protective layer.
In regard to the process aspect the foregoing and other objects are attained by a process for producing corrosion-stable aluminum-based metal pigment manufactured by means of physical vapor deposition (PVD) and subsequent crushing of the metal film produced. The pigment obtained by the crushing operation is treated with one or more substances of the group consisting of carboxylic acids, phosphonic acids, phosphoric acids, phosphomolybdic acid, alcohols, amines, amides and derivatives of said substances. Alternatively a metal oxide and/or metal hydroxide layer is deposited as a passivating protective layer on the free metal surfaces of the pigment obtained by the crushing operation, by hydrolysis of salts or metal acid esters from the group consisting of B, Al, Sn, Ti, V, Cr, Mo, Zn and Ce. Alternatively again an organically modified silicate, titanate, zirconate or aluminium zirconate layer is applied as a passivating protective layer to the free metal surfaces of the pigment obtained by the crushing operation from suitable organic solutions or by hydrolysis of suitably modified metal acid esters. A further alternative is that an organic polymer layer based on acrylates and/or methacrylates is applied as a passivating protective layer to the free metal surfaces of the pigment obtained by the crushing operation by radical polymerization in solution.
As will be seen in greater detail hereinafter from examples the invention provides that all exposed metal surfaces are covered with a very firmly adhering passivating layer with a barrier function. In that respect it is preferred that, in terms of the process of the invention, the passivating layer is applied during the film crushing step, that is to say therefore it is formed in situ directly in the production of fresh fracture edges. The chemically applied passivating protective layer must be of a nature such thatxe2x80x94insofar as the large surfaces of the pigment particles are unprotectedxe2x80x94the protective layer is deposited on the large surfaces of the pigment particles but also in particular covers over the fracture surfaces. As indicated above a number of compounds and operating procedures can be used for that purpose.
Thus the passivating protective layer can comprise substances of the group consisting of long-chain carboxylic acids, phosphonic acids, phosphoric acids, alcohols, amines, amides and derivatives of said substances with between 8 and 20 C-atoms and salt-like compounds of said substances. Those substances may be applied to the pigment particles either from suitable solutions or by direct treatment.
The protective layer however may also comprise at least one metal oxide layer and/or metal hydroxide layer of the group B, Al, Sn, Ti, V, Cr, Mo, Zn and Ce. That layer is precipitated by controlled hydrolysis of suitable salts or metal acid esters.
The protective layer may also comprise organically modified silicates, titanates, zirconates or aluminum zirconates and can also be applied from suitable organic solutions or by hydrolysis of suitably modified metal acid esters.
Another possible protective layer is an organic polymer layer based on acrylates and/or methacrylates which are applied by radical polymerization in solution. That protective layer can be applied to the pigments or in a particular configuration during the actual film crushing operation.
It is possible to conceive of a protective layer which is afforded by combinations of the above-listed substances and procedures.
Tests have also shown that it is advantageous, instead of pure aluminum, to use aluminum alloys with a higher level of resistance to corrosion. Particular emphasis is to be laid on the seawater-resistant aluminum alloy xe2x80x98Hydronaliumxe2x80x99 (with 7% by weight of magnesium and a little silicon) and chromium-bearing aluminum alloys.
Operation is conducted in known manner in the production of the multi-layer pigments. In a roll coater which is a vacuum coating apparatus with an internally disposed foil roll which can be rolled up and unrolled, a foil which is firstly provided with a release layer is successively coated by way of physical vapor deposition with a semi-transparent aluminum layer, for example of between 5 and 40 nm in thickness, then a transparent SiO2-layer for example of between 100 and 600 nm in thickness, then an opaque aluminum layer of  greater than 60 nm in thickness and then again with a transparent SiO2-layer of for example between 100 and 600 nm in thickness and finally with a semi-transparent aluminum layer of for example between 5 and 40 nm in thickness. Instead of pure aluminum, aluminum-based alloys can also be used for the central aluminum layer and/or for the two semi-transparent aluminum layers. Vapor deposition of the aluminum or aluminum-based metal is effected electrically by way of resistance-heated boats or by sputtering. Sputtering is preferred for optically variable pigments. Sputtering is also the preferred method for the vapor deposition of SiO2-layers or SiO2-bearing layers. Mixing SiO2 with cryolite is found to be advantageous as the SiO2/cryolite layer is built up very much more quickly than the build-up of a pure SiO2-layer and cryolite has approximately the same refractive index as SiO2.
To produce the pigment from the multi-layer film, firstly the release layer is dissolved by means of a solvent, the film is peeled off the substrate and the film fragments which are produced in that case are crushed, possibly after washing and filtering. The operation of crushing the film fragments to pigment size is effected by ultrasound or mechanically by high-speed agitators in a liquid medium or after drying thereof in an air jet crusher with a sifter wheel. The free metal surfaces of the pigment which occurs in a particle size of between 5 and 60 xcexcm, preferably between 12 and 36 xcexcm, are coated with a passivating protective layer during the crushing operation or subsequently thereto by one of the above-mentioned processes according to the invention, depending on whether the pigment crushing operation is effected in a liquid medium or dry.
If the passivation operation is effected during the crushing operation, for example carboxylic acid, phosphoric acid or phosphoric acid ester or chromic acid is added to the liquid medium in which the film fragments are present in the crushing treatment. In that respect the medium must have at least a certain solution property for the respectively added passivation agent. Passivation of the dry powder, for example by adding for example metal oxides, polymers, higher fatty acids or phosphoric acid esters in finely divided form is also possible, as well as passivation in the gaseous phase, for example in the procedure for crushing film fragments in an air jet crusher, but it is less preferred. The preferred situation involves using carboxylic acids, phosphoric acids, phosphomolybdic acid or phosphoric acid ester, chromic acid or also mixtures of a plurality of passivation agents in aqueous, alcohol, ketone-type, alkane-type, ether-type or other organic solvents such as tetrahydrofuran, propylacetate or toluene or also mixtures thereof. In regard to the carboxylic acids, higher fatty acids such as stearic acid, oleic acid, myristic acid or for example salts such as for example sodium stearate or zinc stearate are particularly suitable. Dicarboxylic acid or salts thereof can also be used.
Phosphoric acid can also be used in the form of a monobasic or polybasic acid. Among the possible phosphoric acid esters, those based on higher fatty alcohols are particularly preferred. Chromic acid (CrO3) is desirably applied in the form of a 20% aqueous solution.
The concentration of the passivating agent in the liquid medium is selected to be between 5 and 30% by weight under normal circumstances. In the exceptional case of liquid passivation agents it is also possible to operate in a 100% concentrate, thus for example when using oleic acid. The passivating treatment in a liquid medium itself is preferably effected over a period of between 1 and 5 hours. During that period the pigment-bearing suspension is carefully stirred. After the treatment the pigment is filtered off, possibly washed and dried. The need for the washing operation arises only in relation to CrO3-treatment. All experience indicates that a xe2x80x98post-maturing phasexe2x80x99, during which the pigment is exclusively stored for between 3 and 4 weeks, increases the quality of passivation. Analyses show that, depending on the particle size, between 0 and 30% by weight of the passivating agent is firmly xe2x80x98attachedxe2x80x99 to the surface of the pigment.
The corrosion-stable single-layer and multi-layer pigments according to the invention are used for material coloring purposes, in particular for coloring decorative coatings in the lacquer, varnish, paint, plastic material, printing and ceramic sectors.
The boiling test in water described in DE-A 40 30 727 for example serves for quickly checking the effectiveness of passivation, that is to say the water-resistance of the pigments. In that procedure, 1.5 g of the metal or multi-film pigment to be tested is pre-dispersed in the form of paste in 10 g of butyl glycol and then introduced with 150 g of water into a gas-tightly closable apparatus. The mixture is then heated until boiling occurs and the time required until 400 ml of hydrogen is developed is recorded.
Non-stabilised pigments react within a few minutes. The pigments produced according to the invention on the other hand requires boiling times of at least 15 hours until 400 ml of hydrogen is developed.
The following Examples serve to further describe the invention and by means of the above-indicated tests illustrate the achieved passivation and corrosion resistance of the pigments according to the invention.